1. Field of the Invention:
This invention relates to internal flexible or articulated coupling transmissions and, more particularly, to combustion engines with toothed belt synchronous transmissions which transmit a rotational movement from one shaft to another parallel shaft. The shafts are of the type which require mandatory indexing, like those used for controlling overhead camshafts in an internal combustion engine. The installation of one or more overhead camshafts in the cylinder block of a modern internal combustion engine, typically, requires a transmission having two pulley wheels or gears with a ratio of one to one half between which a toothed belt is stretched. Such a transmission is, sometimes, provided in the form of a lubricated sprocket chain or, currently more often, by a synchronous toothed belt, with the wheels or gears connected thereto, in a sealed, dry housing. This technology, which requires indexing the movements of the pulleys, leads to the use of material of low elasticity, preferably glass fiber cable embedded in an elastomer compound, for the belt strap. The teeth of the synchronous belt, or rather a belt for synchronous operation, are molded in the elastomer compound.
2. Description of the Prior Art:
The internal temperature of the engine may range from -35.degree. C. at starting, to continuous temperatures which can stabilize at 110.degree. C. during operation of the engine. That higher value may even be exceeded when the engine is shut down after intensive operation. For proper operation, such engine belts require a moderate belt tension. For example, for an ordinary 4 cylinder engine, a value of 18 deca Newtons ("daN") per belt side is required at current typical operating temperatures.
This tension is increased, in a manner which can be prejudicial to the service life of the belt, by the thermal expansion of the center-to-center distance and the diameter of the shaft supported wheels or gears on which the belts are positioned when the engine reaches a temperature of 110.degree. C. An example of belt tension, actually measured on an engine, was 54 daN per side on a belt having an overall length of approximately one meter. In the same engine as an example, the tension of the belt is reduced to 3.5 daN at the lowest starting temperatures. This reduction in tension, along with the action of the starting torque increased by the viscosity of the oil, can in certain cases cause a dynamic jerk, or skip, which can damage certain parts of the engine.
The principle of synchronous belts with a fixed center-to-center distance allows the use of a tensioning roller pressing against one of the sides of the belt, most frequently on the smooth reverse side of the belt, to establish the tension during installation and during maintenance. A permanent elastic tension mounting of this device is generally difficult and expensive on an internal combustion engine. This is because the engine cycle is, by definition, pulsatory, and the cam follower shaft is also characterized by pulsatory resistance due to the lifting of the valves of the engine. The examination of mass-spring systems, typically, shows highly resonant engine operating conditions accompanied by shocks and high pulsatory fatigue stresses.
Elastic tensioning devices, with incorporated damping, are disclosed in German Patent No. 35 28 442 to Daimler-Benz, European Patent No. 02 43 237 to Hutchinson and European Patent No. 01 88 077 to Litens Automotive. However, the employment of such devices in an overhead camshaft remain difficult and expensive to implement. The manufacturers of such engines are, therefore, required to install belts having a fixed center-to-center distance or, more conventionally, having a tensioning roller which is locked in position after the adjustment has been made. Sometimes there are means to compensate for the variations in center-to-center distance under the action of temperature.
For top-loading and front-loading washing machines, a rather primitive device is described in French Patent No. 2,558,496 to Esswein S.A. The device appears to work by means of a set of differential expansion levers acting on the pulleys in a cantilever fashion. For a transmission which employs a toothed belt, greater demands are placed on the parallelism of the shafts. French Patent No. 2,510,217 to Piaggio discloses a so-called "regulator," which acts by means of an appropriate lever between the aluminum alloy cylinder block expansion and a compensator rod made of steel.
In this apparatus, the tension is initially established by a spring acting on a lever arm. A steel rod, positioned against a stop, limits the force exerted by the spring on the belt when the temperature of the engine block rises, as long as the temperature of the steel rod changes in relation to the engine block temperature.
However, there is no mention in French Patent No. 2,510,217, either of operation at very low temperatures, where the slack in the belt apparently is compensated by the spring alone, or of transient conditions, where the engine block reaches high temperatures much more rapidly than the compensation rod, which stabilizes only several minutes later. A large reduction in the tension of the belt during these transient conditions would likely cause a dynamic jerk, or skipping, of the synchronous belt.